1. Field of the Invention
The embodiments discussed herein relate to a switching power supply device.
2. Background of the Related Art
A switching power supply device that includes a current resonance DC-DC switching converter is suitable to improve its efficiency and to reduce its thickness and thus is employed widely in power supply adapters of televisions and the like. In the current resonance DC-DC switching converter, direct-current voltage obtained by rectifying and smoothing a commercial alternate-current power supply is used as input voltage. In this case, a smoothing capacitor is charged only when the voltage of the alternate-current power supply is high enough to exceed the terminal voltage of the smoothing capacitor, and thus the waveform of input current largely deviates from a sine wave. In this case, as a voltage waveform and a current waveform of alternate current differ from each other, not only the power factor decreases, but a high-frequency current component is generated and becomes a high-frequency noise source. Hence, the switching power supply device includes a power factor correction (PFC) circuit, so that direct-current voltage with less high-frequency noise is used as the input voltage of the current resonance DC-DC switching converter.
The switching power supply device detects this direct-current input voltage and uses the input voltage in various ways. For example, the input voltage is detected to determine whether the commercial alternate-current power supply is a 100 volt system or a 200 volt system. This determination result is used to optimize a detection threshold value of an overcurrent protection circuit, for example. That is, when the commercial alternate-current power supply is a 200 volt system and electric power to be protected is the same, the detection threshold value of overcurrent is set to an approximately half of the detection threshold value set for the 100 volt system.
The input voltage of the current resonance DC-DC switching converter is detected by using a voltage dividing circuit with resistors (for example, refer to Japanese Laid-open Patent Publication No. 9-117144 (paragraph [0032] and FIG. 1)). When the input voltage is detected by using the voltage dividing circuit with the resistors, the electric current always flows through the voltage dividing circuit, and thus detection loss is large, causing deterioration of standby electric power when a load is very small.
To solve this, there is a known configuration for detecting voltage output by a switching element of a current resonance DC-DC switching converter as input voltage (for example, refer to Japanese Laid-open Patent Publication No. 2013-99110 (paragraph [0027] and FIG. 1)). In this Japanese Laid-open Patent Publication No. 2013-99110, when a high side switching element 106 of serially connected switching elements turns on, the voltage of a primary smoothing capacitor 105 is supplied to a series circuit of a resonance reactor and a resonance capacitor 108 and is utilized to detect the input voltage. Specifically, a peak voltage detection circuit (a part of a voltage detection circuit 200) that includes a series circuit of a first resistor 202, a diode 201, and a second resistor 209, and a capacitor 204 connected in parallel with the second resistor 209 is connected to a connection point between the high side switching element and a low side switching element. Terminal voltage of the capacitor 204 of the peak voltage detection circuit is input into a power supply controlling integrated circuit (IC) 110, so that the power supply controlling IC 110 indirectly detects the input voltage (the voltage of the primary smoothing capacitor 105). Electric current flows through the peak voltage detection circuit only when the high side switching element is turned on by the diode 201. Hence, if the high side and low side switching elements perform switching operation with the same ON time ratio (i.e., 50%), electric power consumption is reduced to half as compared with a case in which electric current always flows.
The peak voltage detection circuit described in Japanese Laid-open Patent Publication No. 2013-99110 is configured to hold the voltage of the primary smoothing capacitor when the high side switching element turns on and to cause the power supply controlling IC to detect the held voltage. Hence, the peak voltage detection circuit needs to be arranged outside the power supply controlling IC, thereby increasing the cost for the arrangement. In addition, the power supply controlling IC needs a dedicated input terminal for inputting the held voltage, thereby increasing the cost of the power supply controlling IC. Also, the peak voltage detection circuit is configured to hold peak voltage, and thus its time constant is large, deteriorating input-voltage detection responsiveness. For example, when the input voltage suddenly changes, the input voltage needs to be detected immediately in order to protect the switching power supply device, but a power supply protection function can be disabled when the input-voltage detection responsiveness deteriorates.